The present invention generally relates to a laser beam receiver circuit and, in particular, relates to a photosensitive receiver circuit that determines the position of a laser beam from the sector and the channel of the photosensitive array impinged on by the laser beam.
Rotating laser beams and laser beam receivers are commonly used in agriculture, construction, civil engineering, machine control or surveying applications to obtain positional information about a rotating reference plane of light. Conventionally, positional information is found by sweeping a rotating laser beam over the full length of a long array of photosensors. When the rotating laser beam impinges on one of the photosensors, a signal is provided by the illuminated photosensor which is processed by appropriate circuitry and used to control an operator display or used as an input to machine control circuitry.
Many surveying and engineering applications using laser beam receivers with linear position sensing require that the receivers be relatively long, typically ranging from about six inches up to about six feet, in order for the laser beam to be detected. Additionally, such laser beam receivers may typically have four identical vertical arrays of photosensors, arranged on all four sides of a receiver, to detect laser light coming from any direction. Typically, photosensors are relatively small, and, therefore, are spaced center-to-center about 0.10 inch to about 0.30 inch in a vertical row. With this arrangement a beam of laser light will be detected regardless of the point at which it sweeps across the vertical row of photosensors. Consequently, generally between 100 and 1000 photosensors are required for receivers that are typically used in, for example, engineering and surveying applications. Manufacturing costs and circuit power requirements increase prohibitively with increasing numbers of photosensors, amplifiers and the associated circuitry. Furthermore, due to the large number of photosensors required, an impractical and expensive number of amplifier channels would be needed if each photosensor were to have its own dedicated amplifier channel.
One solution for reducing the cost of components is to group the photosensors, connecting the photosensors to a reduced number of amplifiers as was described in U.S. Pat. No. 5,886,776, issued Mar. 23, 1999. The '776 patent shows a receiver in which photosensors are divided into groups and an amplifier is connected in a repeating pattern to the outputs of non-adjacent photosensors from different groups. The position of the laser beam is determined from the location and strength of the non-zero outputs from the amplifiers. However, incorrect position information might result if the positioning laser beam were to become non-ideal, (i.e., diffused and diffracted by, for example, dust or dirt), resulting in the laser beam impinging with equal strength on more than one group of photosensors. The resulting incorrect position data from the laser beam impinging the non-adjacent photosensors is not the average of the two non-adjacent photosensor readings. Instead, the error in the reported position randomly can become a large percentage of the overall length of the photosensor array.
Another way to reduce the cost of components is to use two separate arrays of photosensors placed adjacent to each other. In this solution, one array connects to the “n” pattern amplifier channels and the other adjacent array connects to the “m” sector amplifier channels. The arrays are electrically isolated. The laser beam then impinges on both the sector and pattern photosensor at the same time, determining unambiguously the position of the beam. However, this design doubles the already large number of photosensors required and increases the size of the laser beam receiver. For these reasons, this solution is impractical.
Still another arrangement is to use only one array of photosensors and to connect the photosensors in both the “m” sector amplifier channels and the “n” pattern amplifier channels simultaneously. This is beneficial, both electrically and mechanically. One buffer amplifier is used for each photsensor so that the outputs of the buffer amplifiers are connected into the respective sets of “n” pattern amplifiers and “m” sector amplifiers through sufficient impedance to keep to a minimum the signal cross-talk between the two separate sets of amplifiers. The major drawback to this solution would be that it would require a large number of buffer amplifiers in addition to the number “m” plus “n” amplifier channels. Again this solution is impractical.
Therefore, a need exists for an improved laser beam receiver comprised of non-interdigitated photosensors divided into a repeating patterns of “n” channels of photosensors connected to smaller groups of “m” sectors where the photocurrent from both the anode and cathode terminals of the photosensor are used. In this arrangement, no additional photosensors are needed to provide a sector signal. In addition, buffer amplifiers are not needed because the photocurrent from both terminals of each photosensor is used. As a result, the cost to produce the array is significantly reduced. There is an additional need to determine more accurately the position of the laser beam by mathematical interpolation of the relative strength of the laser beam power on each of the photosensors. Further, there is also a need for a receiver that is less susceptible to producing erroneous position data when the laser spot quality is non-ideal, e.g., diffused by dust or dirt.